The research proposed here is designed to clarify certain aspects of the structure and development of the retino-geniculo-cortical pathway in the cat. Four sets of experiments are planned: 1) Possible neurotransmitters in the retinogeniculate pathway will be investigated using light and electron microscopic immunohistochemistry and transmitter-specific retrograde labeling; 2) The effects of impulse blockade on the morphological development of retinogeniculate synapses will be studied in kittens that are reared with intraocular injections of tetrodotoxin; 3) Golgi methods will be used to reveal the effects of monocular deprivation and deafferentation on the dendritic development of neurons in the dorsal lateral geniculate nucleus (LGN); 4) The projection of the LGN to the lateral suprasylvian visual area in newborn and early postnatal kittens will be mapped using horseradish peroxidase (HRP) retrograde tracing methods, and it will be determined in adult cats that have had visual cortex removed at birth whether LGN relay cells that project to the lateral suprasylvian visual area receive direct synaptic input from the retina. The long-term objective of this research is to improve understanding of some of the mechanisms underlying the effects of visual deprivation and amblyopia, and in addition to shed some light on the modes of reorganization employed by the visual system to compensate for damage. These goals are approached by studying normal structure and development in concert with experiments designed to measure the direct effects of abnormal visual experience or brain injury.